ICU-23055 Fix broken links in docs

[sh-cho]

Fixed broken links in the docs. Also fixed links like https://unicode-org.github.io/icu/userguide/strings/index because it makes sidebar closed and erase current position.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-23055
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[CLAassistant]

All committers have signed the CLA.

[sven-oly]

I apologize for being so slow to reply to this. Thank you!

[echeran]

@sh-cho Thank you so much for this PR! It needs one simple update from you and then it will be ready to merge. In the CI checks area of the PR, the single-commit job name is hyperlinked. If you click that, it goes to a page. Check the checkbox and click the squash button. That's it. (The commit message title field should be prepopulated with something that begins with "ICU-23055 ", such as "ICU-23055 Fix broken links in docs", and that prefix is important.)

I already updated a lot of the ticket administrative information and preformed the merge conflict resolution due to upstream changes, so all of that should be ready to go.

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

~ Your Friendly Jira-GitHub PR Checker Bot